1. Field of the Invention
The present invention generally relates to a multi-hop relay Broadband Wireless Access (BWA) communication system, and in particular, to an apparatus and method for providing information about a relay link zone in which information to be relayed to a Mobile Station (MS) is sent.
2. Description of the Related Art
Providing services with diverse Quality of Service (QoS) requirements at or above 100 Mbps to users is an active study area for a future-generation communication system called a 4th Generation (4G) communication system. In particular, active research is being conducted on providing high-speed service by ensuring mobility and QoS to a BWA communication system, such as Wireless Local Area Network (WLAN) systems, Wireless Metropolitan Area Network (WMAN) systems, etc. Such major examples comply with Institute of Electrical and Electronics Engineers (IEEE) 802.16d and IEEE 802.16e standards.
IEEE 802.16d and IEEE 802.16e communication systems are implemented by applying Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) to physical channels. IEEE 802.16d considers only a single-cell structure with no regard to mobility of Subscriber Stations (SSs). In contrast, IEEE 802.16e supports mobility of SSs to the IEEE 802.16d communication system. A mobile SS is referred to below as an MS.
FIG. 1 shows a conventional IEEE 802.16e communication system configured in a multi-cell structure. The system shown in FIG. 1 includes cells 100 and 150, BSs 110 and 140 for managing the cells 100 and 150, respectively, and a plurality of MSs 111, 113, 130, 151 and 153. Signaling is carried out in OFDM/OFDMA between the BSs 110 and 140 and the MSs 111, 113, 130, 151 and 153. The MS 130 exists in a cell boundary area between the cells 100 and 150, i.e. in a handover region. When the MS 130 moves to the cell 150 managed by the BS 140 during signal transmission/reception to/from the BS 110, the serving BS of the MS 130 changes from the BS 110 to the BS 140.
Since signaling in FIG. 1 is performed between an MS and a fixed BS via a direct link, a highly reliable radio communication link can be established between them in the conventional IEEE 802.16e communication system. However, due to the fixedness of BSs, a wireless network cannot be configured with flexibility. As a result, the IEEE 802.16e communication system is not effective in efficiently providing communication services under a radio environment experiencing a fluctuating traffic distribution and a great change in the number of calls.
The above problem can be solved by applying a multi-hop relay data transmission scheme using fixed Relay Stations (RSs), mobile RSs, or general MSs to general cellular wireless communication systems, such as IEEE 802.16e. A multi-hop relay wireless communication system can advantageously reconfigure a network rapidly according to a communication environmental change and enables efficient operation of the whole wireless network. For example, a multi-hop relay wireless communication system can expand cell coverage and increase system capacity. When the channel status between a BS and an MS is bad, an RS can be installed between them so the resulting establishment of a multi-hop relay path through the RS renders a higher-speed radio channel available to the MS. With the use of a multi-hop relay scheme at a cell boundary offering a bad channel status, high-speed data channels can be provided and the cell coverage can be expanded.
Now a description will be made of the configuration of the multi-hop relay wireless communication system which expands cell coverage of BSs.
FIG. 2 shows a conventional multi-hop relay BWA communication system configured to expand the cell coverage of BSs. The multi-hop relay BWA communication system, which is configured in a multi-cell structure, includes cells 200 and 240, BSs 210 and 250 for managing the cells 200 and 240, respectively, a plurality of MSs 211 and 213 within the coverage area of the cell 200, a plurality of MSs 221 and 223 managed by the BS 210 but located in an area 230 outside the cell 200, an RS 220 for providing multi-hop relay paths between the BS 210 and the MSs 221 and 223 within the area 230, a plurality of MSs 251, 253 and 255 within the coverage area of the cell 240, a plurality of MSs 261 and 263 managed by the BS 250 but located in an area 270 outside the cell 240, and an RS 260 for providing multi-hop relay paths between the BS 250 and the MSs 261 and 263 within the area 270. OFDM/OFDMA signals are exchanged among the BSs 210 and 250, the RSs 220 and 260, and the MSs 211, 213, 221, 223, 251, 253, 255, 261 and 263.
Although the MSs 211 and 213 within the coverage area of the cell 200 and the RS 220 can communicate directly with the BS 210, the MSs 221 and 223 within the area 230 cannot communicate with the BS 210 directly. Therefore, the RS 220 covering the area 230 relays signals between the BS 210 and the MSs 211 and 223. Meanwhile, although the MSs 251, 253 and 255 within the coverage area of the cell 240, and the RS 260 can communicate directly with the BS 250, the MSs 261 and 263 within the area 270 cannot communicate with the BS 250, directly. Therefore, the RS 260 covering the area 270 relays signals between the BS 250 and the MSs 261 and 263.
In the multi-hop relay BWA communication system shown in FIG. 2, the RSs 220 and 260 are infrastructure RSs installed by service providers and are thus known to the BSs 210, 240 and 310, or client RSs acting as SSs or MSs, or as RSs under circumstances. The RSs 220 and 260 may also be fixed, nomadic (e.g. laptop), or mobile like MSs.
To expand cell coverage by use of an RS as described above, a conventional frame structure defined for communications between a BS and an MS should be extended to enable communications among a BS, an MS and an RS. In other words, a frame structure should be defined, which enables a BS to communicate with a plurality of RSs and MSs based on a single communication platform. To do so, a DownLink (DL) frame from the BS should be divided into a BS-MS communication zone and a BS-RS communication zone and an UpLink (UL) frame to the BS should be divided into an MS-BS communication zone and an RS-BS communication zone. That is, the RSs should be accommodated through appropriate division of limited resources. However, considering that the number of RSs connected to the BS and the channel environment are time-variant, i.e. the cell environment is very variable, it is inefficient to fix the BS-RS communication zone or the RS-BS communication zone. In this context, techniques for dynamically operating these zones have recently been proposed. Accordingly, a need exists for defining a signaling procedure for providing information about the zones to an RS.